This invention relates to a load-current monitor circuit for a MOS power transistor and more particularly to such a circuit in which the gate and the source respectively of a small MOS monitor transistor are connected to the gate and source of the power transistor.
Load-current monitor circuits for a MOS power transistor in the prior art typically have the circuit configuration shown in FIG. 1. A large area MOS power or driver transistor 10 is connected in series with the load 12 forming a circuit branch that is connected between a voltage supply terminal 14 and a circuit ground point 15. A much smaller area MOS monitor transistor 18, having been made simultaneously and by the same process steps used for making the large driver transistor 10 has identical characteristics, e.g. the same channel-polarity and the same threshold voltage, as does the driver transistor 10. The large high current MOS driver transistor 10 is usually made up of an array of smaller elemental transistors. The small MOS transistor 18, hereinafter referred to as a monitor transistor, is usually made up using only one or two such small but additional elemental transistors.
The drains of the driver transistor 10 and the monitor transistor 18 are connected to the voltage supply terminal 14 and the gates of the driver and monitor transistors are both connected to the driver input terminal, namely the driver control-voltage terminal 20. Current Im through the monitor transistor 18 flows to ground through a MOS transistor and an NPN transistor 24. A differential amplifier 26 has one input connected to the source of the driver transistor 10 and the other input connected to the source of the monitor transistor 18. The output of the amplifier 26 is connected to the gate of the transistor 22 for forcing the source voltage Vms of the monitor transistor 18 to be more nearly equal to the source voltage Vds of the driver transistor 10. Since the characteristics of the driver and monitor transistors are the same, when the sources of the driver and monitor transistor are indeed equal, the relatively small monitor current Im is proportional to the load current flowing through the driver transistor 10. When these conditions are realized in practice, the current ratio Id/Im is essentially exactly equal to the ratio of the area of the driver transistor 10 to the area of the monitor transistor 18. The monitor output current Iout from the collector of the current mirror transistor 28 would consequently be proportional to the load current.
This circuit provides an output current Iout that tracks the load current Im rather well, for high values of load current. However, the lower the load current becomes, the poorer that tracking becomes, and in fact when the load current approaches zero, the monitor output current may be quite poorly or even unrecognizably related to the load current.
Circuits of this kind are often employed as a current regulator by feeding back the monitor output current to the driver control input. Regulation will then be good only at relatively high currents.
For some applications, accuracy at low driver current levels is also required. For example, servo motors typically operate most of the time at slow speeds. Thus tracking accuracy between monitor current and driver load current at low motor drive currents is particularly important if not critical in a servo application.
It is an object of this invention to provide a driver load current monitor that overcomes the aforementioned shortcomings of prior art.
lt is a further object of this invention to provide a driver load current monitor wherein the monitor output current is proportional to the driver load current down to near zero current levels.
It is yet another object of this invention to provide such a driver load current monitor wherein the monitor output current is essentially zero when the driver load current is zero.